1. Field of the Invention
This invention relates to a process for recovering protein from an animal muscle source and to the product so-obtained. More particularly, this invention relates to a process for recovering muscle proteins from an animal source and to the protein product so-obtained.
2. Description of Prior Art
Presently, there is an interest in expanding the use of muscle proteins as food because of their functional and nutritional properties. Better use of these materials would be particularly important with low value raw materials for which there is currently little or no human food use. These raw materials include the fatty, pelagic fish and deboned muscle tissue from fish and poultry processing. However, the use of these materials has been hampered because of the loss of functionality of the proteins during processing, the instability of the product due to lipid oxidation, and unappealing characteristics such as dark colors, strong flavors, unsightly appearance and poor texture. Protein functionalities of most concern to food scientists are solubility, water holding capacity, gelation, fat binding ability, foam stabilization and emulsification properties. A considerable effort has been spent to produce a protein concentrate from under utilized fish species. This effort has met with only limited success. In one example, it was thought necessary to remove the lipids by an organic solvent extraction process to stabilize the product. This is not only expensive and requires recycling of the solvent, but it has the serious problem of destroying the functional properties of the protein. As a nutritional supplement, it can not compete in cost against the proteins from soy and its poor solubility and water-binding characteristics prevents it from being added as a functional component in most products.
In an alternative approach, protein concentrates from muscle tissue, especially fish, have been made by hydrolysis. This approach has improved some functional properties, particularly solubility, which has allowed its use in prepared soups. However, this approach also destroys other functional properties such as gelling ability. The raw materials that can be used in these products are limited due to sensitivity to undesirable lipid oxidation. Thus, at the present time, moderate success has only been achieved with relatively expensive lean, white fleshed fish as the source of the animal protein.
One process that has had some success in stabilizing protein foods has been the process for producing "surimi". This has been used primarily for fish, although there have been some attempts to produce a surimi-like product from other raw materials such as deboned poultry mince. In producing surimi, the muscle is ground and washed with a variable amount of water a variable number of times. This is determined by the location of the plant and the product that is desired from the particular species. Water may be used in a ratio as low as about 2 parts water to one part fish up to about 5 parts water per 1 part fish; typically about 3 parts water are used per 1 part fish. The number of washes can vary, generally, from 2 to 5, again depending on the raw material, the product desired, and water availability. Twenty to thirty per cent of the fish muscle proteins are solubilized when the ground muscle is washed with water. These soluble proteins, known as sarcoplasmic proteins, are generally not recovered from the wash water of the process. This loss is undesirable since sarcoplasmic proteins are useful as food. The washed minced product containing the protein in solid form then is used to make protein gels. Originally, this was used to produce "kamaboko" in Japan. Kamaboko is a popular fish sausage in which the washed minced fish is heated until it gels.
It is presently believed that it is necessary to add cryprotectants to the washed, minced fish before freezing to prevent protein denaturation. A typical cryoprotectant mixture comprises about 4% sucrose,about 4% sorbitol and about 0.2% sodium tripolyphosphate. These components retard the denaturation of the protein during freezing, frozen storage and thawing. High quality surimi has generally only been produced from lean white fish. Much effort has been made into determining how to make a quality product from dark-fleshed, pelagic fatty species. As discussed above, these species as a protein source have limitations based on stability to lipid oxidation, color, poor gelling ability, low yields, and the necessity for using very fresh raw material. The most successful Japanese process for producing a surimi from a dark-fleshed fish loses about 50-60% of the total protein of the muscle tissue. It also can have color and lipid stability problems.
It has been proposed by Cuq et al, Journal of Food Science, pgs. 1369-1374 (1995) to provide edible packaging film based upon fish myofibrillar proteins. In the process for making the films, the protein of water-washed fish mince is solubilized in an aqueous acetic acid solution at pH 3.0 to a final concentration of 2% protein. This composition has a sufficiently high viscosity because of the use of acetic acid so that membranes could not be separated by the procedure of this invention. The viscosity of these solutions was further increased by the addition of 35 g of glycerol per 100 g of dry matter to obtain sufficiently high solution viscosities so that films could be formed. These compositions contain insufficient concentrations of water to avoid highly viscous solutions or gels. Thus, the undesirable non-protein fractions including membrane lipids which affect product quality can not be removed from the protein fraction. In addition, the use of acetic acid imparts a strong odor to the material which would severely limit its use in a food product.
It also has been proposed by Shahidi and Onodenalore, Food Chemistry, 53 (1995) 51-54 to subject deboned, whole capelin to washing in water followed by washing in 0.5% sodium chloride, followed by washing in sodium bicarbonate. The series of washes, including that using sodium bicarbonate, would remove greater than 50% of the muscle proteins. Essentially all of the sarcoplasmic proteins would be removed. Final residue was further washed to remove residual bicarbonate. The washed meat was then suspended in cold water and heated at 70.degree. C. for 15 min. This heat treatment is sufficient to "cook" the fish proteins, thus denaturing them and reducing or eliminating their functional properties. The dispersion is centrifuged at 2675.times. g for 15 minutes and the protein in the supernatant is determined at pH between 3.5 and 10.0. The dispersion required heating at 100.degree. C. to reduce the viscosity. The reduced viscosity, however, was still much greater than is achieved with the process of this invention. The resultant suspensions of Shahidi and Onodenalore were sufficiently concentrated so that membrane lipids cannot be separated from the protein by centrifugation.
Shahidi and Venugopal, Journal of Agricultural and Food Chemistry 42 (1994) 1440-1448 disclose a process for subjecting Atlantic herring to washing in water followed by washing with aqueous sodium bicarbonate. Again, this process will remove greater than 50% of the muscle proteins, including the sarcoplasmic proteins. The washed meat was homogenized and the pH varied between 3.5 and 4.0 with acetic acid. As mentioned above, the acetic acid produces a highly viscous suspension under these conditions and does not permit separation of membrane lipids from proteins by centrifugation. In addition, there is an odor problem with the volatile acetic acid.
Venugopal and Shahidi, Journal of Food Science, 59, 2 (1994) 265-268, 276 also disclose a process for treating minced Atlantic mackerel suspended in water and glacial acetic acid at a pH of 3.5. This gives a material that is too viscous to permit separation of membrane lipids from protein by centrifugation. It also has the odor problem caused by acetic acid.
Shahidi and Venugopal, Meat Focus International, October 1993, pgs 443-445 disclose a process for forming homogenized herring, mackerel or capelin in aqueous liquids having a pH as low as about 3.0. It is reported that acetic acid reduces the viscosity of herring dispersions, increases viscosity of mackerel dispersions to form a gel and precipitates capelin. All of these preparations were initially washed with sodium bicarbonate, which would remove a substantial proportion of the protein, including the sarcoplasmic proteins. No process step is disclosed which permits separation of proteins from membrane lipids.
Accordingly, it would be desirable to provide a process for recovering a high proportion of available muscle protein from an animal source. It would also be desirable to provide such a process,which permits the use of muscle protein sources which are presently underutilized as a food source such as fish having a high fat or oil content. Furthermore, it would be desirable to provide such a process which recovers substantially all of the protein content of the process feed material. In addition, it would be desirable to provide such a process which produces a stable, functional, protein product which is particularly useful for human consumption.